1. Field of the Invention
This invention relates generally to the field of seismic prospecting and more particularly to impact type surface seismic sources where in a heavy mass is propelled downwardly to impact an earth-contacting base plate, thereby generating a seismic pulse of acoustic frequency.
2. Description of the Prior Art
As described in U.S. Pat. No. 4,284,165 issued Aug. 18, 1981 to Tom P. Airhart et al, entitled Acoustic Pulse Generator, apparatus for generating an acoustic pulse employs a hollow upstanding housing having a circular, longitudinal bore open at its lower end and closed at its upper end. An elongated piston extends slidably within this bore and terminates at its bottom end in a heavy impact mass. Latch means support the piston and the means in spaced relation above an earth-contacting base plate. A firing signal actuates the latch means so as to release the mass, enabling the piston to drop within the housing bore. In so doing it exposes the upper piston surface to high pressure air entering the bore through sidewall ports provided for that purpose. This air pressure forcibly propels the mass downwardly to strike the baseplate, which couples the transferred kinetic energy into the earth as a seismic impulse. The contents of the afore-mentioned patent are incorporated herein by reference.
The operation of the above described seismic source is improved by the use of means such as described in U.S. Pat. No. 4,402,381 issued Sept. 6, 1983 to Tom P. Airhart and entitled Vehicle Mounting and Deployment Arrangement for Seismic Source. According to this patent, also incorporated herein by reference, adjustable vertical supports consisting of single rod-end hydraulic cylinders are positioned between the vehicle chassis and the base plate. When the baseplate is deployed at a particular location the cylinders are operated like jacks to transfer the weight of the vehicle, at least in part, so that it rests upon and keeps the baseplate from shifting position under repeated blows from the impact mass. The bottom ends of the piston rods are not physically attached to the base plate but instead are provided with transverse bearing plates which are guided into contact with the upper base plate surface as the rods are extended. This guidance is provided by means of upstanding posts fixed to the base plate which make sliding contact with the edges of the bearing plates. When the base plate is to be picked up from the earth for redeployment or transport, these bearing plates conveniently engage lift rings also mounted upon and fastened to the baseplate.
The present invention is designed to further improve upon the base plate alignment means taught in U.S. Pat. No. 4,402,381 in order to achieve superior results on uneven terrain and with varying modes of operation. In order to appreciate the importance of such alignment in the generation of seismic waves, it is necessary to analyze the way in which such waves are produced. In generating seismic waves by impacting a base plate, the angle between the path of the descending mass and the plane of the base plate as it contacts the earth determines the relative proportions of compressional and shear waves so generated. When, for example, the apparatus is used to generate compressional waves only, the path of the descending mass must be perpendicular to the plane of the base plate even if it rests on a non-horizontal earth surface. Alternatively, the generator may be operated so that the mass strikes the base plate along a tilted or inclined path or along opposing inclined paths. Such a mode of operation is described in commonly assigned copening U.S. Patent application Ser. No. 6-676762 filed Nov. 3, 1984, in the name of Tom P. Airhart entitled, Mounting and Control Means for Full Wave Form for Seismic Source. In such case the null position of the actuator axis from which the necessary angular tilts are reckoned must be at right angles to the base plate plane. Otherwise the presumed proportions of compressional and shear wave components in the resultant seismic wave will not be present. Furthermore, if the base plate slides or translates along the earth's surface under a slanting blow such as described above, the vehicle will have to be shifted so that the actuator is again correctly positioned over the center of the base plate. If not, successive wave forms will differ in energy content, the base plate may be damaged, and other components of the system may be heavily stressed.